Bone change with age and disease, manifested in modification of mineral content, density, and structure, are evident in loss of mechanical properties which can be detected by ultrasonics. The Less-Davidson hypothesis asserts, on the basis of the stereo-chemical structure of collagen, that bone matrix stiffens as a consequence of the mineralization process. Furthermore, a linear dependence between bone density and sonic velocity has been found for normal adult bone. Defective bone collagen is known to be associated with weak bone and the sonic velocity must be significantly less than for normal bone. Elastic properties, dimensional changes with water content, density, and composition of osteoporotic, osteodystrophic, and osteomalacic bone will be compared to determine if there are differences that can be attributed to defects in the collagen intermolecular structure. Bones of animals having known pathologies induced under controlled conditions will be examined by methods including ultrasonic microscopy to relate cross-linking collagen defects to decreases in density and ultrasonic velocity. Insight and understanding of the fundamental processes controlling the elastic properties of bone at the molecular and ultrastructural level is sought. It will contribute to the theory of the properties of bone and how it is affected by age and pathologies like osteoporosis, renal osteodystrophy, and Paget's disease.